Well... Though phpBB is free and all, people buy forums for certain merits and achievements. Some think when operating a forum that was purchased makes them cooler and feel that they are serious about running whatever.
I personally operate and support phpBB 100% and I am happy with that. Though truth be told I have an IPB License, i don't operate it, but I surely do have it.
Though the one thing that made me stick with phpBB, not only is it free, it's the massive amount of support it gets from fellow users around with common or similar interest. Plus phpBB can practically do anything a user wants, as long as the user can respect and learn to use it properly.